Could this be the "Crash Dive Blues," i.e. did you crash dive in order to escape the destroyers? If you did, and you issue a new depth order before reaching the actual crash dive depth, you will not be able to maintain depth at ahead slow. I don't think that this should be ruled out until the possibility is eliminated.
So in your example, you could hit the C key and then order 200 meters before reaching the crash dive depth. If you have done this, then you are experiencing the "Crash Dive Blues."
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